WO2024117236A1 - Method for recovering alkali metal salt and apparatus for recovering alkali metal salt - Google Patents

Abstract

The present invention relates to a method for recovering an alkali metal salt, said method comprising steps 1 and 2 below. Step 1 is a first nanofiltration step in which a solution X containing alkali metal ions is fed, as a to-be-treated liquid A, to a nanofiltration membrane unit A and is thereby separated into a liquid permeate A and a concentrated liquid B, then the concentrated liquid B is mixed with the residue of the to-be-treated liquid A and the result thereof is fed to the nanofiltration membrane unit A again, and more of the liquid permeate A is obtained. Step 2 is a second nanofiltration step in which: the liquid permeate A obtained in step 1 or a concentrated liquid of the liquid permeate A is fed, as a to-be-treated liquid B, to the nanofiltration membrane unit A and is thereby separated into a liquid permeate C and a concentrated liquid D, then the concentrated liquid D is mixed with the residue of the to-be-treated liquid B and the result thereof is fed to the nanofiltration membrane unit A again, and more of the liquid permeate C is obtained; or alternatively, the liquid permeate A obtained in step 1 or a concentrated liquid of the liquid permeate A is fed, as a to-be-treated liquid B, to a nanofiltration membrane unit B and is thereby separated into a liquid permeate C and a concentrated liquid D, then the concentrated liquid D is mixed with the residue of the to-be-treated liquid B and the result thereof is fed to the nanofiltration membrane unit B, and more of the liquid permeate C is obtained.

Description

Method and apparatus for recovering alkali metal salts

The present invention relates to a method for recovering alkali metal salts and an apparatus for recovering alkali metal salts.

In recent years, the demand for mineral resources has increased significantly in line with the development of the global economy. For example, there is growing demand for lithium as a material for lithium-ion batteries, and lithium carbonate is also used as an additive for heat-resistant glass and for surface acoustic wave filters. High-purity lithium carbonate in particular is used as filters and transmitters in mobile phones and car navigation systems.

Cobalt is also widely used in various industries as an alloying element for special steels and magnetic materials. For example, special steels are used in the aerospace, power generator and special tools fields, and magnetic materials are used in small headphones and small motors. Cobalt is also used as a raw material for the positive electrode material of lithium-ion batteries, and demand for cobalt is increasing with the spread of mobile information processing devices such as smartphones, as well as batteries for automobiles and power storage.

Nickel is used in stainless steel, taking advantage of its luster and high corrosion resistance, and in recent years, like cobalt, demand for it as a material in lithium-ion batteries has been on the rise. As demand for various rare metals increases, efforts are being made to recover rare metals such as lithium, cobalt, and nickel from used lithium-ion batteries and waste materials generated during the manufacturing process, with the aim of recycling valuable resources.

For example, efforts are underway to recover resources from used lithium-ion batteries, primarily rare metals such as cobalt and nickel, but the mainstream method is solvent extraction using a chelating agent, which places a large burden on the environment and is also disadvantageous in terms of cost (Non-Patent Document 1). To solve this problem, a separation and recovery method has been disclosed (Patent Document 1) that uses separation membranes such as ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes from an aqueous solution obtained by leaching used lithium-ion batteries with acid. However, in this separation and recovery method, the nanofiltration membrane is a one-stage process, so it is difficult to recover lithium at a high purity and in a large amount unless the nanofiltration membrane performance is extremely high.

Therefore, a separation and recovery method that uses nanofiltration membranes in multiple stages has been disclosed (Patent Document 2). This method is a continuous process in which the liquid that has passed through the nanofiltration membrane is passed through the nanofiltration membrane again to improve the lithium purity, and the liquid that has not passed through the nanofiltration membrane is passed through the nanofiltration membrane again to recover the remaining lithium.

International Publication No. 2019/018333 International Publication No. 2021/215484

"Report on the FY2017 Mineral Resource Infrastructure Development Survey for Exploration and Other Businesses to Promote Mineral Resource Development (Basic Survey on the Formulation of a Mineral Resource Securing Strategy)," Mitsubishi Research Institute, Inc., Environment and Energy Business Division, March 2018.

However, the method described in Patent Document 2 involves a complex and continuous process, so there is a risk that the nanofiltration membrane processing step may become unstable if the composition of the liquid being processed changes or if the separation performance changes due to deterioration of the nanofiltration membrane, etc., and there is room for improvement in terms of maintaining a specified lithium purity and recovery rate.

The object of the present invention is to provide a method for recovering alkali metal salts stably, with high purity and in large quantities, using a small number of steps, from lithium ion batteries and waste materials, waste liquids, ores, etc., generated during the manufacturing process of the batteries.

In order to achieve the above object, the present invention has the following configuration.
(1) A method for recovering an alkali metal salt, comprising the following steps 1 and 2:
Step 1: A first nanofiltration step in which a solution X containing alkali metal ions is sent to a nanofiltration membrane unit A as a treated liquid A, and separated into a permeated liquid A and a concentrated liquid B, and further, the concentrated liquid B is mixed with the remainder of the treated liquid A and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid A.
Step 2: A second nanofiltration step in which the permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit A as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D, and the concentrate D is mixed with the remainder of the liquid to be treated B and sent to the nanofiltration membrane unit A again to obtain the permeated liquid C, or the permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit B as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D, and the concentrate D is mixed with the remainder of the liquid to be treated B and sent to the nanofiltration membrane unit B again to obtain the permeated liquid C.
(2) A method for recovering an alkali metal salt, in which a process for obtaining the permeate C from the solution X through the step 1 and the step 2 is sequentially performed on N solutions X (N: an integer of 2 or more), in which the step 2 uses the nanofiltration membrane unit B, and while the step 1 is performed on a k-th solution X(k) (k: an integer of 1 to (N-1)) among the N solutions X and then the step 2 is being performed, the step 1 is performed in parallel on a (k+1)-th solution X(k+1), the method for recovering an alkali metal salt described in (1) above.
(3) The method for recovering an alkali metal salt according to (1) or (2) above, further comprising a step of diluting at least one of the liquid A to be treated and the liquid B to be treated.
(4) The method for recovering an alkali metal salt according to (2) or (3) above, further comprising the following step 3:
Step 3: A reverse osmosis filtration step for concentrating at least one of the kth permeate A(k) and the kth permeate C(k) in at least one of said solutions X(k).
(5) The method for recovering an alkali metal salt according to (4) above, wherein the step 3 is carried out only once on the permeated liquid C(k).
(6) The method for recovering an alkali metal salt according to any one of (1) to (5) above, wherein the solution X has a pH of 4 or less.
(7) The method for recovering an alkali metal salt according to any one of (1) to (6) above, wherein the alkali metal ions include lithium ions.
(8) A method for recovering an alkali metal salt according to any one of (1) to (7) above, comprising the following step 4:
Step 4: a step of adding a remainder of the kth liquid to be treated B(k) having been mixed with the kth concentrated liquid D(k) to the mth solution X(m) (m: an integer of 2 or more) or the mth liquid to be treated A(m) after completion of step 2 for the kth solution X(k) (k: an integer of 1 or more and (N-1) or less) among the N solutions X, to the mth solution X(m) (m: an integer of (k+1) or more and N or less) or the mth liquid to be treated A(m).
(9) The nanofiltration membrane of at least one of the nanofiltration membrane unit A and the nanofiltration membrane unit B has a porous support membrane and a separation functional layer,
A positron beam is irradiated from the surface of the nanofiltration membrane on the side of the separation functional layer, and the average pore size R1 and the average pore size R2 of the separation functional layer derived by a positron annihilation lifetime measurement method satisfy 0.90≦R1/R2≦1.10.
A method for recovering an alkali metal salt according to any one of (1) to (8) above.
R1: average pore size under a condition of a positron beam intensity of 0.1 keV, R2: average pore size under a condition of a positron beam intensity of 0.5 keV (10) At least one of the steps 1 and 2 is carried out at a constant permeation flow rate, and at least one of the steps 1 and 2 is terminated when the recovery rate A (%) of the alkali metal ions reaches a target value based on the following formula (2) while monitoring the change over time of the operating pressure.

[In formula (2), A is the alkali metal ion recovery rate (%), P is the operating pressure (Pa), P _is the initial operating pressure (Pa), V _is the initial liquid volume to be treated (m ³ ), R is the alkali metal ion removal rate (%) of the nanofiltration membrane, S is the liquid recovery rate (%) of the nanofiltration process, Q _F is the supply flow rate (m ³ /s), Q _c is the concentrated liquid flow rate (m ³ /s), and t is the end time of filtration (t=tb)]
(11) At least one of the permeated liquid A(k) and the permeated liquid C(k) contains neutral molecules that are not charged under conditions of pH 3 or less, and the reverse osmosis filtration membrane used in the reverse osmosis filtration step is a low-removal reverse osmosis membrane that has an isopropyl alcohol removal rate of 70% or more and less than 85% when an aqueous isopropyl alcohol solution having a pH of 6.5 and an operating pressure of 0.5 MPa at 25° C. is permeated therethrough. The method for recovering an alkali metal salt according to any one of (4) to (10).
(12) The method for recovering an alkali metal salt according to (11) above, wherein the neutral molecule is a boron compound.
(13) The method for recovering an alkali metal salt according to (12) above, further comprising a circulation step of mixing the concentrated solution obtained in the reverse osmosis filtration step with a solution to be supplied to the reverse osmosis filtration step in the step 3.
(14) The method for recovering an alkali metal salt according to any one of (11) to (13) above, comprising a step of feeding the permeated liquid obtained in the reverse osmosis filtration step to a high-removal reverse osmosis membrane unit equipped with a high-removal reverse osmosis membrane having an isopropyl alcohol removal rate of 85% to 95% when an aqueous isopropyl alcohol solution having a pH of 6.5 and an operating pressure of 0.5 MPa is passed through the high-removal reverse osmosis membrane, and adding the permeated liquid obtained as dilution water to at least one of the liquid A to be treated and the liquid B to be treated.
(15) A first separation means for separating a solution containing alkali metal ions as a treatment liquid A into a permeate liquid A and a concentrate liquid B by a first nanofiltration membrane unit;
a first circulation means for mixing the concentrated liquid B with the remainder of the liquid A to be treated;
a second separation means for separating the permeated liquid A or a concentrate of the permeated liquid A as a liquid to be treated B into a permeated liquid C and a concentrate D by a second nanofiltration membrane unit;
A second circulation means for mixing the concentrated liquid D with the remainder of the liquid to be treated B;
A dilution means for adding dilution water to at least one of the liquid A and the liquid B;
a flow rate control means capable of controlling the flow rates of the permeated liquid A and the concentrated liquid B in the first separation means, and the permeated liquid C and the concentrated liquid D in the second separation means;
An alkali metal salt recovery apparatus comprising: a flow control means for synchronizing the flow rate of dilution water added in the dilution means with the flow rate of the permeated liquid when the treated liquid to which the dilution water is added is sent to a nanofiltration membrane unit.

The method for recovering alkali metal salts of the present invention makes it possible to stably recover high-purity, high-volume salts of alkali metals such as lithium and cesium from solutions containing alkali metal ions with a small number of steps.

FIG. 1 is a schematic flow diagram showing a method for recovering an alkali metal salt according to one embodiment of the present invention. FIG. 2 is a schematic flow diagram showing a method for recovering an alkali metal salt according to another embodiment of the present invention. FIG. 3 is a schematic flow diagram showing a method for recovering an alkali metal salt according to another embodiment of the present invention. FIG. 4 is a schematic flow diagram showing a method for recovering an alkali metal salt in a comparative example. FIG. 5 is a schematic flow diagram showing a method for recovering metal salts according to another embodiment of the present invention. FIG. 6 is a schematic flow diagram showing a method for recovering an alkali metal salt in a comparative example. FIG. 7 is a schematic flow diagram showing a method for recovering an alkali metal salt in a comparative example. FIG. 8 is a schematic flow diagram showing a method for recovering alkali metal salts according to another embodiment of the present invention. FIG. 9 is a schematic flow diagram showing a method for recovering an alkali metal salt according to another embodiment of the present invention. FIG. 10 is a schematic flow diagram showing a method for recovering alkali metal salts according to another embodiment of the present invention.

The following describes in detail an embodiment of the present invention, but the present invention is not limited to the following description, and can be modified and implemented as desired without departing from the spirit of the present invention.

(1) Method for recovering alkali metal salt The method for recovering alkali metal salt of the present invention is a method for recovering an alkali metal salt from a solution containing alkali metal ions, and includes the following steps 1 and 2.
Step 1: A first nanofiltration step in which a solution X containing alkali metal ions is sent to a nanofiltration membrane unit A as a treated liquid A, and separated into a permeated liquid A and a concentrated liquid B, and further, the concentrated liquid B is mixed with the remainder of the treated liquid A and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid A.
Step 2: A second nanofiltration step in which the permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit A as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D, and the concentrate D is mixed with the remainder of the liquid to be treated B and sent to the nanofiltration membrane unit A again to obtain the permeated liquid C, or the permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit B as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D, and the concentrate D is mixed with the remainder of the liquid to be treated B and sent to the nanofiltration membrane unit B again to obtain the permeated liquid C.

The method for recovering alkali metal salts according to this embodiment is a method for recovering alkali metal salts, in which the process of obtaining the permeate C from the solution X through the steps 1 and 2 is carried out sequentially for N solutions X (N: an integer of 2 or more), and it is preferable that the nanofiltration membrane unit B is used in the step 2, and that while the step 2 is being carried out after the step 1 is carried out for the kth solution X(k) (k: an integer of 1 to (N-1)) among the N solutions X, the step 1 is carried out in parallel for the (k+1)th solution X(k+1). A series of processing steps in which the batch processing steps in steps 1 and 2 are carried out semi-continuously for a plurality of solutions is called a semi-batch processing step.

(2) Nanofiltration Step In the nanofiltration step, a nanofiltration membrane is used to separate a solution containing alkali metal ions into a permeate and a concentrate.

The ratio of alkali metal ion concentration to polyvalent metal ion concentration in the permeate (hereinafter referred to as the "alkali metal ion ratio") is higher than the alkali metal ion ratio in solution X, and the alkali metal ion ratio in the concentrated solution is lower than the alkali metal ion ratio in solution X.

The concentration of polyvalent metal ions is calculated as the sum of the ion-equivalent concentrations of, for example, cobalt ions and nickel ions. The concentration of alkali metal ions is calculated as the sum of the ion-equivalent concentrations of, for example, lithium ions and cesium ions. Some alkali metal elements may exist in a solution as polyatomic ions rather than as monoatomic ions, but the converted concentrations are those assuming they exist as monoatomic ions. The concentrations of the above polyvalent metal ions and alkali metal ions can be determined, for example, by analyzing the solution to be measured using a Hitachi P-4010 ICP (inductively coupled plasma atomic emission spectrometry) device to quantify the concentrations (mg/L) of various ions.

(2-1) Solution containing alkali metal ions The solution X containing alkali metal ions may contain at least alkali metal ions and one or more conjugate bases (e.g., chloride ions, nitrate ions, sulfate ions, carbonate ions, acetate ions, etc.). The alkali metal ions and conjugate bases in solution X may be present in the form of alkali metal salts, and examples of the alkali metal salts include salts of lithium, sodium, potassium, rubidium, and cesium. Among these, it is preferable to contain a salt of lithium from the viewpoint of the value of the object to be recovered. That is, it is preferable that the solution containing alkali metal ions contains lithium ions (hereinafter also referred to as "Li ⁺ ") as the alkali metal ions.

When there are N solutions X containing alkali metal ions, each solution X only needs to contain at least an alkali metal ion and one or more conjugate bases, and the composition of the solution, such as the alkali metal ion concentration, the polyvalent metal ion concentration, and the pH, may differ for each solution X. In particular, it is preferable that all solutions X contain Li ⁺ as the alkali metal ion.

The solution X containing alkali metal ions preferably contains at least one polyvalent metal ion in addition to the alkali metal ions. Examples of polyvalent metal ions include alkaline earth metals such as magnesium, calcium, and strontium, typical elements (aluminum, tin, lead, etc.), and transition elements (iron, copper, cobalt, manganese, etc.).

Also, the solution X containing alkali metal ions may contain neutral molecules that are not charged under conditions of pH 3 or less, and the neutral molecules preferably have a molecular weight of 70 or less. Examples of neutral molecules include boron compounds such as formic acid, acetic acid, and boric acid. Among them, boron compounds may be added as additives to the electrolyte of lithium ion batteries in order to improve the characteristics of the battery, and therefore may be included in the solution X containing alkali metal ions. For example, if a boron compound is included, it will become a purification inhibitor during lithium recovery, but it can be removed in the reverse osmosis filtration process described below. The boron concentration (mg/L) in the solution X is preferably equal to or less than the alkali metal ion concentration of the target to be recovered, more preferably equal to or less than 0.5 (mg/L) times the alkali metal ion concentration of the target to be recovered, and even more preferably equal to or less than 0.1 (mg/L) times the alkali metal ion concentration of the target to be recovered.

The solution X containing alkali metal ions preferably has a pH of 0 or more and 4 or less.
The pH of the solution X containing alkali metal ions is preferably 4 or less, more preferably 3.5 or less, even more preferably 3 or less, and even more preferably 2.5 or less. By having a pH of 4 or less, the permeability of the alkali metal ions is increased while the permeability of the polyvalent metal ions is maintained low in the nanofiltration step.
Moreover, the pH of the solution X containing alkali metal ions is preferably 0 or more, more preferably 0.5 or more, and even more preferably 1 or more. By having a pH of 0 or more, it is possible to suppress a decrease in the selective separation performance of the nanofiltration membrane for alkali metal ions relative to polyvalent metal ions during long-term operation.

The solution X containing alkali metal ions is preferably a solution in which a material containing lithium is dissolved in an acid. Specific examples of the material containing lithium include lithium ion batteries and waste materials, waste liquids, ores, and slags generated in the manufacturing process thereof. Among these, lithium ion batteries are preferred because of the high demand for reuse and the high purity of the rare metals contained therein.
A lithium ion battery is composed of components such as a positive electrode material, a negative electrode material, a separator, and an electrolyte. Any of these components containing lithium can be used as the material of solution X. The acid for dissolving the lithium-containing material preferably contains at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, and nitric acid. The solution obtained by dissolving the components of a lithium ion battery with an acid contains, in addition to lithium ions, for example, nickel, cobalt, manganese, and the like.

A method for dissolving an alkali metal-containing substance with an acid includes, for example, immersing the substance in an acidic aqueous solution. However, other methods may be used as long as the desired alkali metal ions can be eluted. The temperature of the acidic aqueous solution to be contacted is preferably 10°C or higher and 100°C or lower from the viewpoint of the elution efficiency of the alkali metal ions. Moreover, from the viewpoint of cost and safety, it is more preferably 20°C or higher and 80°C or lower.
A solution in which an alkali metal-containing substance is dissolved in an acid does not always have a constant composition, but may vary in composition due to variations in the composition of various ions in the substance and dissolution conditions in the acid, etc. In other words, when there are N solutions X, the compositions of the N solutions X may differ from one another.

There is no particular restriction on the volume of solution X containing the alkali metal ions to be recovered, and when there are N solutions X, the volumes of the N solutions X may be different. From the viewpoint of the processing efficiency in each step, it is preferable that the volume of solution X is 10 L or more and 10,000 L or less.

The solution X containing alkali metal ions may contain organic compounds. For example, when solution X is an acid solution for a lithium-ion battery, examples of organic compounds include polyvinylidene fluoride (PVDF), polyolefins, and carbonates derived from the binder, separator, electrolyte, etc. that connects the active material to the current collector. These organic compounds may act as foulants and cause a decrease in the recovery efficiency of alkali metal ions, so these foulants may be removed by an ultrafiltration process described below.

When solution X containing N alkali metal ions each contains lithium ions as the alkali metal ions, it is preferable that the lithium ion concentration in the solution is 0.5 mg/L or more and 10,000 mg/L or less. By having a lithium ion concentration in the solution of 0.5 mg/L or more, the efficiency of lithium ion recovery by membrane separation is improved. Furthermore, by having a lithium ion concentration in the solution of 10,000 mg/L or less, the osmotic pressure difference is prevented from becoming large, and the efficiency of membrane separation is improved. The lithium ion concentration in the solution is more preferably 10 mg/L or more and 8,000 mg/L or less, and even more preferably 100 mg/L or more and 6,000 mg/L or less.

The method for recovering alkali metal salts according to this embodiment can also be suitably used when the alkali metal ion ratio of solution X containing alkali metal ions is 2.4 or less. Generally, when the alkali metal ion ratio is 2.4 or less, it becomes more difficult to separate and recover alkali metal ions and polyvalent metal ions. However, the method for recovering alkali metal salts according to this embodiment has high selective separation between alkali metal ions and polyvalent metal ions, and can effectively recover alkali metal ions. Furthermore, the method for recovering alkali metal salts according to this embodiment can also be suitably used when the alkali metal ion ratio of solution X containing alkali metal ions is 1 or less, and even 0.5 or less.

(2-2) Nanofiltration Membrane The nanofiltration membrane used in the alkali metal salt recovery method according to this embodiment may have a fractionation characteristic positioned between a reverse osmosis membrane and an ultrafiltration membrane. The difference between the glucose removal rate when a 1000 mg / L glucose aqueous solution at 25 ° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa and the isopropyl alcohol removal rate when a 1000 mg / L isopropyl alcohol aqueous solution at 25 ° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa is preferably 20% or more. Among them, the nanofiltration membrane has a glucose removal rate when a 1000 mg / L glucose aqueous solution at 25 ° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa, and a difference between the glucose removal rate and the isopropyl alcohol removal rate when a 1000 mg / L isopropyl alcohol aqueous solution at 25 ° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa. The glucose removal rate is 70% or more, and the magnesium sulfate removal rate when a 2000 mg / L magnesium sulfate aqueous solution at 25 ° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa is 95% or more. Hereinafter, when the term "glucose removal rate" is used simply in this specification, it means the glucose removal rate when a 1000 mg/L aqueous glucose solution at 25° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa; when the term "isopropyl alcohol removal rate" is used, it means the isopropyl alcohol removal rate when a 1000 mg/L aqueous isopropyl alcohol solution at 25° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa; and when the term "magnesium sulfate removal rate" is used, it means the magnesium sulfate removal rate when a 2000 mg/L aqueous magnesium sulfate solution at 25° C. and pH 6.5 is permeated at an operating pressure of 0.5 MPa.
Membranes commonly known as reverse osmosis are capable of removing most organics and ions, whereas ultrafiltration membranes typically do not remove most ionic species and remove high molecular weight organics.

In order to separate alkali metal ions and polyvalent metal ions, it is preferable that the nanofiltration membrane has a charge on the membrane surface, and both separation by pores (size separation) and electrostatic separation by charge are possible. For example, in a nanofiltration membrane in which the difference between the glucose removal rate and the isopropyl alcohol removal rate is 40% or more and the glucose removal rate is 70% or more, and in addition, the difference between the magnesium sulfate removal rate and the magnesium chloride removal rate when a 2000 mg/L magnesium chloride aqueous solution at 25°C and pH 6.5 is passed through at an operating pressure of 0.5 MPa is 20% or less, both size separation and electrostatic separation are possible. Hereinafter, when the term "magnesium chloride removal rate" is simply used in this specification, it means the magnesium chloride removal rate when a 2000 mg/L magnesium chloride aqueous solution at 25°C and pH 6.5 is passed through at an operating pressure of 0.5 MPa.

Materials for nanofiltration membranes include polymers such as cellulose acetate polymers, polyamides, sulfonated polysulfones, polyacrylonitrile, polyesters, polyimides, and vinyl polymers. Nanofiltration membranes may be made of only one type of material, or may be made of multiple materials. The membrane structure may be an asymmetric membrane with a dense layer on at least one side of the membrane, with gradually increasing pore sizes from the dense layer toward the inside of the membrane or toward the other side, or a composite semipermeable membrane with a very thin separation functional layer made of a different material on top of the dense layer of the asymmetric membrane.

The composite semipermeable membrane is preferably, for example, a membrane having a porous support membrane containing polysulfone and a separation functional layer containing polyamide provided on the porous support membrane. The composite semipermeable membrane may also have a substrate in addition to the porous support membrane and the separation functional layer, in which case the porous support membrane is provided on the substrate. The polyamide is a thin film formed on the porous support membrane by an interfacial polycondensation reaction between a polyfunctional aliphatic amine and a polyfunctional aromatic acid halide.

In the method for recovering an alkali metal salt according to this embodiment, the nanofiltration membrane of at least one of the nanofiltration membrane unit A and the nanofiltration membrane unit B has a porous support membrane and a separation functional layer, and the average pore size R1 and the average pore size R2 of the separation functional layer derived by positron annihilation lifetime measurement method by irradiating a positron beam from the surface of the nanofiltration membrane on the separation functional layer side preferably satisfy 0.90≦R1/R2≦1.10. Here, R1 and R2 are defined as follows.
R1: Average pore diameter when the positron beam intensity is 0.1 keV R2: Average pore diameter when the positron beam intensity is 0.5 keV

The "positron annihilation lifetime measurement method" measures the time (on the order of hundreds of picoseconds to tens of nanoseconds) between when a positron enters a sample and when it annihilates, and non-destructively evaluates information such as the size, number density, and size distribution of 0.1-10 nm vacancies based on the annihilation lifetime.

The measurement range in the depth direction from the sample surface can be adjusted by the amount of energy of the positron beam incident on the sample. The higher the energy, the deeper the measurement range from the sample surface, but the depth depends on the density of the sample. For example, when measuring the separation functional layer of a composite semipermeable membrane, if a positron beam with an energy of about 0.1 keV is irradiated from the separation functional layer side of the composite semipermeable membrane, a region 1.0 to 5.0 nm deep from the sample surface is usually measured, and if a positron beam with an energy of about 0.5 keV is used, a region 10 to 50 nm deep from the sample surface is usually measured. If other layers such as a protective layer are provided on the separation functional layer, the average pore size of the separation functional layer can be measured by removing the other layers beforehand.

In this embodiment, the thickness of the separation functional layer in the composite semipermeable membrane is preferably 15 nm or more and 50 nm or less. Therefore, when the positron beam intensity is 0.1 keV, it reflects the average pore size on the surface side (opposite the porous support membrane side) of the separation functional layer, and when it is 0.5 keV, it reflects the average pore size on the porous support membrane side of the separation functional layer, and it can be said that the closer R1/R2 is to 1, the more uniform the pore size is in the film thickness direction. It is presumed that the uniform pore size in the film thickness direction makes the direction in which ions diffuse in the separation functional layer uniform, and the permeation resistance of monovalent ions, which are of a size that can move freely in the separation functional layer, is suppressed. As a result, it is believed that excellent monovalent ion/multivalent ion selective separation performance is realized. Therefore, it is more preferable that R1/R2 is 0.92 to 1.05, and even more preferable that it is 0.94 to 1.03.

R1 is preferably 0.55 nm or more and 0.70 nm or less, more preferably 0.57 nm or more and 0.68 nm or less, and even more preferably 0.60 nm or more and 0.65 nm or less. When R1 is within the above range, the effect of inhibiting the permeation of polyvalent metal ions while suppressing the permeation resistance of alkali metal ions is remarkable.
In order for R1 and R2 to satisfy the above relationship, for example, the relative humidity during the interfacial polycondensation of a polyfunctional aliphatic amine compound and a polyfunctional aromatic acid halide described later is controlled to be high, for example, 80% or more, and the molecular weight of the polyfunctional aliphatic amine that forms the separation functional layer in the composite semipermeable membrane is set to 90 or more.

The separation functional layer in the composite semipermeable membrane preferably contains 50% by mass or more of semi-aromatic crosslinked polyamide obtained by interfacial polycondensation of a divalent or higher polyfunctional aliphatic amine compound and a divalent or higher polyfunctional aromatic acid halide, more preferably 80% by mass or more, and even more preferably 90% by mass or more, and is particularly preferably composed of only semi-aromatic crosslinked polyamide. By containing 50% by mass or more of semi-aromatic crosslinked polyamide, excessive densification due to π-π interactions derived from aromatic rings in the semi-aromatic crosslinked polyamide is suppressed, and excellent alkali metal ion permeability is obtained. Furthermore, as a result of intensive research, the present inventors have found that when the relative humidity during the interfacial polycondensation is controlled to a high level, for example, 80% or more, the obtained composite semipermeable membrane exhibits particularly excellent membrane performance under acidic conditions. The relative humidity can be adjusted by using a precision air conditioning device, etc. By setting the atmospheric humidity during the interfacial polycondensation to 80% or more, evaporation of water from the formed polyamide can be suppressed, and insolubilization due to intermolecular hydrogen bonds of oligomers with many amino groups generated in excess can be suppressed. This allows the oligomers to be efficiently removed after the separation functional layer is formed by the interfacial polycondensation reaction, suppressing the enlargement of the pore size associated with the swelling of the semi-aromatic crosslinked polyamide during membrane operation under acidic conditions, and is believed to result in a composite semipermeable membrane that exhibits excellent polyvalent ion removal properties, i.e., a specified glucose removal rate, isopropyl alcohol removal rate, magnesium sulfate removal rate, etc.

The polyfunctional aliphatic amine is preferably an alicyclic diamine, and more preferably a bipiperidine derivative or a piperazine derivative.

The molecular weight of the alicyclic diamine is preferably 90 or more. When the molecular weight of the alicyclic diamine is 90 or more, the diffusion coefficient of the amine is small, and polyamide is gradually formed during interfacial polycondensation, so that a separation functional layer with a uniform pore size in the film thickness direction is easily formed from the initial to middle stages of interfacial polycondensation. On the other hand, the molecular weight of the alicyclic diamine is preferably 160 or less. Usually, at the initial and final stages of interfacial polycondensation, oligomers are excessively generated on the support surface in contact with the organic layer, and the pores on the support surface are blocked, which causes the pore size distribution in the film thickness direction to become non-uniform. However, when the molecular weight of the alicyclic diamine is 160 or less, the molecular weight of the generated oligomers is small, and the interaction with the semi-aromatic crosslinked polyamide can be reduced, so that after the separation functional layer is formed by the interfacial polycondensation reaction, the oligomers are easily detached from the separation functional layer, and a separation functional layer with a uniform pore size in the film thickness direction is easily formed.

Examples of alicyclic diamines with a molecular weight of 90 to 160 include substituted piperazines in which the piperazine ring is substituted with an alkyl group having 1 to 3 carbon atoms (e.g., 2-methylpiperazine, 2-ethylpiperazine, 2-normalpropylpiperazine, 2,2-dimethylpiperazine, 2,2-diethylpiperazine, 2,3-dimethylpiperazine, 2,3-diethylpiperazine, 2,5-dimethylpiperazine, 2,5-diethylpiperazine, 2,6-dimethylpiperazine, 2,6-diethylpiperazine, 2,3,5,6-tetramethylpiperazine, etc.) and homopiperazine.

The term "polyfunctional aromatic acid halide" refers to an aromatic acid halide having two or more halogenated carbonyl groups in one molecule, and is not particularly limited as long as it gives a semi-aromatic crosslinked polyamide upon reaction with the polyfunctional aliphatic amine. Examples of polyfunctional aromatic acid halides that can be used include halides of 1,3,5-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic acid, 1,3-benzenedicarboxylic acid, 1,4-benzenedicarboxylic acid, 1,3,5-benzenetrisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid. Among the polyfunctional aromatic acid halides, acid chlorides are preferred, and in terms of economy, ease of availability, ease of handling, ease of reactivity, and the like, trimesoyl chloride, which is an acid halide of 1,3,5-benzenetricarboxylic acid, isophthaloyl chloride, which is an acid halide of 1,3-benzenedicarboxylic acid, terephthaloyl chloride, which is an acid halide of 1,4-benzenedicarboxylic acid, 1,3,5-benzenetrisulfonic acid chloride, which is an acid halide of 1,3,5-benzenetrisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid chloride, which is an acid halide of 1,3,6-naphthalenetrisulfonic acid, are particularly preferred. The above polyfunctional aromatic acid halides may be used alone or in a mixture of two or more kinds, but by mixing the trifunctional trimesic acid chloride, 1,3,5-benzenetrisulfonic acid chloride, or 1,3,6-naphthalenetrisulfonic acid chloride with either the bifunctional isophthalic acid chloride or terephthalic acid chloride, the intermolecular gaps of the polyamide crosslinked structure are expanded, and a membrane with a uniform pore size distribution can be controlled over a wide range. The molar ratio of the trifunctional acid chloride to the bifunctional acid chloride is preferably 1:20 to 50:1, and more preferably 1:1 to 20:1.

The above composite semipermeable membrane can be obtained, for example, by forming a porous support membrane on a substrate, and then forming a separation functional layer containing a semi-aromatic crosslinked polyamide on the porous support membrane by interfacial polycondensation of a polyfunctional aliphatic amine and a polyfunctional aromatic acid halide.

(2-3) Separation by nanofiltration membrane Since alkali metal ions easily permeate nanofiltration membranes and polyvalent metal ions have difficulty permeating nanofiltration membranes, it is possible to separate alkali metal ions from polyvalent metal ions. The nanofiltration membrane is preferably used in a state where it is incorporated into an element such as a spiral type.

(2-3-1) Step 1: First Nanofiltration Step The first nanofiltration step (step 1) is a step in which the solution X is sent to the nanofiltration membrane unit A as the treated liquid A, separated into the permeated liquid A and the concentrated liquid B, and the concentrated liquid B is mixed with the remaining part of the treated liquid A and sent to the nanofiltration membrane unit A again to obtain the permeated liquid A. In step 1, the concentrated liquid B can be treated two or more times with the nanofiltration membrane unit A. The number of times this treatment is repeated can be set arbitrarily. In addition, the repeated treatment in step 1 may be terminated when the recovery rate reaches a certain value, as described later. By permeating the treated liquid A through the nanofiltration membrane unit A while mixing the concentrated liquid B with the remaining part of the treated liquid A, the alkali metal ions remaining in the concentrated liquid B can be permeated again through the nanofiltration membrane, making it possible to increase the recovery rate of the alkali metal ions.
As the first nanofiltration process progresses, the amount of permeated liquid A increases, the recovery rate (%) of alkali metal ions increases, and the amount of treated liquid A and the ratio of alkali metal ions in treated liquid A decrease.

In the first nanofiltration step, it is preferable to obtain a permeate having an alkali metal ion ratio of 2 or more and 1000 or less, more preferable to obtain a permeate having an alkali metal ion ratio of 10 or more and 700 or less, and even more preferable to obtain a permeate having an alkali metal ion ratio of 2 or more and 500 or less. Having an alkali metal ion ratio of 2 or more makes it possible to shorten the processing time of the subsequent second nanofiltration step, and having an alkali metal ion ratio of 1000 or less makes it possible to shorten the processing time of this step.

In addition, in the first nanofiltration step, the recovery rate of alkali metal ions is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. If the recovery rate of alkali metal ions in the first nanofiltration step is 80% or more, the cost of recovering alkali metal ions can be reduced. The recovery rate of alkali metal ions in the nanofiltration step is defined for each step by the following formula (1).

　Alkaline metal ion recovery rate (%) in nanofiltration process = {(volume of permeate in target nanofiltration process) x (concentration of alkali metal ions in permeate in target nanofiltration process)} / {(volume of liquid to be treated) x (initial concentration of alkali metal ions in liquid to be treated)} ... formula (1)

In the nanofiltration process, it is preferable to supply the solution to the nanofiltration membrane at an operating pressure in the range of 0.1 MPa to 8 MPa. If the operating pressure is 0.1 MPa or more, the membrane permeation rate improves, and if it is 8 MPa or less, damage to the nanofiltration membrane can be suppressed. It is more preferable that the operating pressure is 0.5 MPa to 6 MPa, and even more preferable that it is 1 MPa to 4 MPa.

In the first nanofiltration step, the osmotic pressure of the treated liquid A increases as the step proceeds, and the operating pressure required to obtain the same flow rate of the permeated liquid A increases accordingly. Therefore, the method for recovering an alkali metal salt according to this embodiment preferably includes a step of diluting the treated liquid A in order to facilitate continuation of the filtration when the osmotic pressure increases. Diluting the treated liquid A is preferable because it reduces the osmotic pressure of the treated liquid A, allowing the first nanofiltration step to be continued and increasing the recovery rate of the alkali metal ions.
Examples of the method for diluting the liquid to be treated A include a method of directly adding dilution water to the liquid to be treated A and a method of adding dilution water to the concentrated liquid B. Among these, the method of directly adding dilution water to the liquid to be treated A is preferred because it is simple.

The dilution water may be pure water, an acidic solution, or the like, but is not limited to this. However, it is preferable to use the permeate with a low metal ion concentration produced in the reverse osmosis filtration process described below, as this allows for efficient separation and recovery of alkali metal ions and allows the acidic aqueous solution to be reused.

The operation control method in the first nanofiltration step is not particularly limited, and may be, for example, constant flow rate filtration, low pressure filtration, etc., but constant flow rate filtration is preferred when diluting the treated liquid A. With constant flow rate filtration, the flow rate of the dilution water added can also be kept constant, making control easier.
In the case of constant flow rate filtration, from the viewpoint of Li ⁺ recovery efficiency, it is preferable that the permeate flow rate is 1% or more of the volume of solution X per minute, and from the viewpoint of ease of control, it is preferable that the permeate flow rate is 50% or less of the volume of solution X per minute.

The degree of progress of the first nanofiltration step, i.e., the recovery rate of alkali metal ions, can be known by appropriately sampling the treated liquid A and analyzing the liquid composition. However, since analyzing the liquid composition takes time, it is preferable to be able to constantly monitor the recovery rate of alkali metal ions.
As a method for monitoring the recovery rate of alkali metal ions during the progress of the first nanofiltration step, when the nanofiltration step is performed at a constant permeation flow rate, there is a correlation between the operating pressure (operation pressure) and the recovery rate of alkali metal ions as shown in the following formula (2). Therefore, it is preferable to grasp the recovery rate A (%) of alkali metal ions while monitoring the change over time of the operating pressure using the following formula, and determine the end time of the first nanofiltration step. By determining the end time of the first nanofiltration step while monitoring the operating pressure using formula (2), the time required for analyzing the liquid composition can be reduced, and alkali metal ions can be efficiently recovered. The target recovery rate A (%) of alkali metal ions can be appropriately set. At least one of steps 1 and 2 can be terminated on the condition that the recovery rate reaches a target value.

In the above formula (2), A is the alkali metal ion recovery rate (%), P is the operating pressure (Pa), P _is the initial operating pressure (Pa), V _is the initial liquid volume to be treated (m ³ ), R is the alkali metal ion removal rate (%) of the nanofiltration membrane, S is the liquid recovery rate (%) of the nanofiltration process, Q _is the supply flow rate (m ³ /s), Q _is the concentrated liquid flow rate (m ³ /s), and t is the end time of filtration (t=tb).

The liquid recovery rate S of the nanofiltration process is defined as S={( _QF - _Qc )/ _QF }x100.

1 to 3, 5, and 8 to 10 are schematic flow diagrams showing a method for recovering an alkali metal salt according to one embodiment of the present invention. Solution X is sent to an ultrafiltration membrane unit 1, which will be described later, and the resulting permeate is sent to a first tank 5a. The permeate (liquid A to be treated) stored in the first tank 5a is sent to a nanofiltration membrane unit A (2a) and separated into permeate A and concentrated liquid B. Permeate A is sent to a second tank 5b at a constant flow rate, and concentrated liquid B is sent to the first tank 5a and mixed with the remaining part of the liquid A to be treated in the first tank 5a. The first nanofiltration step can also be performed while adding dilution water to the first tank 5a at the same flow rate as that of permeate A. The dilution water may include permeate obtained by a reverse osmosis filtration step, which will be described later. In the example shown in FIG. 3 and FIG. 9, the permeate obtained by the reverse osmosis filtration step, which will be described later, is further sent to a high-removal reverse osmosis membrane unit 4, and the resulting permeate is used as dilution water.

(2-3-2) Step 2: Second nanofiltration step In the second nanofiltration step (step 2), the permeated liquid A obtained in the first nanofiltration step (step 1) or the concentrated liquid of the permeated liquid A obtained in step 1 is sent to the nanofiltration membrane unit A as the treated liquid B, separated into the permeated liquid C and the concentrated liquid D, and further mixed with the remaining part of the treated liquid B and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid C; or the permeated liquid A or the concentrated liquid of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit B as the treated liquid B, separated into the permeated liquid C and the concentrated liquid D, and further mixed with the remaining part of the treated liquid B and sent again to the nanofiltration membrane unit B to further obtain the permeated liquid C. In step 2, the concentrated liquid D can also be treated two or more times with the nanofiltration membrane unit A or the nanofiltration membrane unit B. The number of times this process is repeated can be set arbitrarily. Furthermore, the repeated treatment in step 2 may be terminated when the recovery rate reaches a certain value, as described above.
The concentrated solution of permeate A can be prepared by concentrating permeate A using a reverse osmosis membrane unit described below, but there is no particular limitation to this.
In the second nanofiltration step, from the viewpoint of shortening the treatment time, it is preferable to use the permeated liquid A as the liquid to be treated B, and it is also preferable to use the nanofiltration membrane unit B.

In the second nanofiltration step, it is preferable to obtain a permeate C having an alkali metal ion ratio of 10 or more, more preferable to obtain a permeate C having an alkali metal ion ratio of 100 or more, and even more preferable to obtain a permeate C having an alkali metal ion ratio of 200 or more. If the alkali metal ion ratio is 10 or more, it can be said that the purity of the alkali metal ions is sufficiently high.
In the second nanofiltration step, the recovery rate of the alkali metal ions is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. If the recovery rate of the alkali metal ions in the nanofiltration step of the permeate is 80% or more, the cost of recovering the alkali metal can be reduced.

The second nanofiltration step is carried out at least once. If the alkali metal ion ratio of solution X is small and the alkali metal ion ratio of permeate C does not reach the target value when the second nanofiltration step is carried out once, the second nanofiltration step may be carried out multiple times with permeate C as treated liquid B until the alkali metal ion ratio of the resulting permeate reaches the target value. When the second nanofiltration step is carried out multiple times, the nanofiltration membrane unit used from the second time onwards may be either nanofiltration membrane unit A or B, or another nanofiltration membrane unit may be used.

As with the first nanofiltration process, the second nanofiltration process also preferably includes a step of diluting the treated liquid B. In addition, the progress of the process can be grasped using the above formula (2).

Examples of the operation control method in the second nanofiltration step include constant flow rate filtration and low pressure filtration.
In the case of constant flow rate filtration, from the viewpoint of Li ⁺ recovery efficiency, it is preferable that the permeate flow rate is 1% or more of the initial volume of the treated liquid B per minute, and from the viewpoint of ease of control, it is preferable that the permeate flow rate is 50% or less of the initial volume of the treated liquid B per minute.

In the examples shown in Figures 1, 3, 8 and 9, the nanofiltration process of solution X is performed in nanofiltration membrane unit A (2a), and the permeated liquid A that has permeated nanofiltration membrane unit A and is stored in second tank 5b is sent to third tank 5c as treated liquid B, the treated liquid B is sent to nanofiltration membrane unit B (2b), and the permeated liquid C is sent to fourth tank 5d at a constant flow rate to perform a second nanofiltration process. At this time, concentrated liquid D that did not permeate nanofiltration membrane unit B (2b) is mixed with the remainder of treated liquid B in third tank 5c. In addition, the second nanofiltration process can be performed while adding dilution water to treated liquid B in third tank 5c at the same flow rate as permeated liquid C.

In the example shown in Figures 5 and 10, a nanofiltration process is performed on solution X in nanofiltration membrane unit A (2a), and permeated liquid A that has permeated nanofiltration membrane unit A and is stored in second tank 5b is sent to first tank 5a as treated liquid B and then sent again to nanofiltration membrane unit A (2a), and permeated liquid C is sent to second tank 5b at a constant flow rate to perform a second nanofiltration process. At this time, concentrated liquid D that did not permeate nanofiltration membrane unit A (2a) is mixed with the remainder of treated liquid B in first tank 5a. Also, the second nanofiltration process can be performed while adding dilution water to treated liquid B in first tank 5a at the same flow rate as permeated liquid C.

In Figures 5 and 10, the second nanofiltration step uses the same nanofiltration membrane unit A (2a) as the first nanofiltration step, so it is preferable to wash the nanofiltration membrane unit A (2a), the first tank 5a, and the second tank 5b before carrying out the second nanofiltration step.

Furthermore, in the example shown in FIG. 8, the remainder of the liquid to be treated B(k) remaining in the third tank 5c after the kth solution X(k) has been treated is added to the first tank 5a containing the pth liquid to be treated A(p). Furthermore, in the example shown in FIG. 10, the remainder of the liquid to be treated B(k) remaining in the first tank 5a after the kth solution X(k) has been treated is added to the first tank 5a containing the mth liquid to be treated A(m). Here, m is an integer between (k+1) and N, and p is an integer between (k+2) and N.

In the example shown in FIG. 2, the liquid that has passed through the nanofiltration membrane unit A (2a) and is stored in the second tank 5b is sent to the fifth tank 5e, and then the liquid in the fifth tank 5e is sent to the first reverse osmosis membrane unit 3a to carry out the reverse osmosis filtration process described below, and the resulting concentrated liquid is sent to the sixth tank 5f. The concentrated liquid stored in the sixth tank 5f is sent to the third tank 5c, and the liquid in the third tank 5c is sent to the nanofiltration membrane unit B (2b) as the liquid to be treated B, the permeated liquid C is sent to the fourth tank 5d at a constant flow rate, and the concentrated liquid D is mixed with the remaining part of the liquid to be treated B in the third tank 5c, and the second nanofiltration process is carried out while adding dilution water to the liquid to be treated B in the third tank 5c at the same flow rate as the permeated liquid C.

(3) Semi-batch processing process As described above, the liquid composition of solution X may vary, so in a process in which N (N: an integer of 2 or more) solutions X containing alkali metal ions are continuously pumped, it is very difficult, and in some cases impossible, to stabilize the liquid composition obtained in the nanofiltration membrane processing process and maintain a predetermined lithium purity and recovery rate. In addition, if a continuous process is realized to eliminate such instability in the nanofiltration membrane processing process and enable high-purity and high-recovery lithium, the number of nanofiltration membrane stages will be excessive, resulting in problems of increased costs and further complication of the process. In addition, in a batch processing process, the processing of the kth (k: an integer of 1 to (N-1)) solution X(k) is completed before the processing of the next (k+1)th solution X(k+1) is started, which causes a problem of long processing time.

The alkali metal salt recovery method according to this embodiment is a method for recovering alkali metal salts, in which the process of obtaining a permeate C from the solution X through steps 1 and 2 is carried out sequentially for N solutions X (N: an integer of 2 or more), and it is preferable that in step 2, a nanofiltration membrane unit B is used, and while a first nanofiltration step (step 1) is carried out for the k-th solution X(k) (k: an integer of 1 to (N-1)) among the N solutions X and then a second nanofiltration step (step 2) is carried out, a first nanofiltration step (step 1) is carried out in parallel for the (k+1)th solution X(k+1) among the N solutions X. In this way, by configuring the batch processing steps in steps 1 and 2 as semi-batch processing steps that are carried out semi-continuously for a plurality of solutions, alkali metal ions can be efficiently recovered.

In addition, in order to efficiently recover alkali metal ions, it is preferable to add the remainder of treated liquid B(k) after completion of the second nanofiltration step of solution X(k) to solution X (m: an integer equal to or greater than (k+1) and equal to or less than N) or treated liquid A(m) and carry out the first nanofiltration step of solution X(m).

In this case, in the second nanofiltration step, the recovery rate of the alkali metal is preferably 50% or more and 95% or less, more preferably 60% or more and 90% or less, and even more preferably 70% or more and 85% or less.
In the second nanofiltration step, by setting the recovery rate of the alkali metal to 95% or less, the permeation of the permeate having a low alkali metal ratio in the later stage of the nanofiltration step can be suppressed. For the purpose of suppressing the permeation of the permeate having a low alkali metal ratio in the later stage of the nanofiltration step, it is preferable not to add dilution water in the second nanofiltration step.

The remainder of the treated liquid B(k) is a liquid obtained by filtering solution X(k) once through a nanofiltration membrane, and has a higher alkali metal ratio than solution X(m) if the compositions of solutions X(m) and X(k) do not vary significantly. Therefore, by adding the remainder of the treated liquid B(k) to solution X(m) or to the treated liquid A(m), the entire amount of the alkali metal in the remainder of the treated liquid B(k) can be recovered, and the alkali metal ratio of solution X(m) is also improved, which in turn improves the alkali metal ratio of the permeate from the first nanofiltration step of solution X(m). In other words, the purity and recovery rate of the alkali metal are improved.

Since the remainder of the treated liquid B(k) is a liquid that has been through the ultrafiltration process, it is more preferable from the standpoint of reducing the load of the ultrafiltration process to add the remainder of the treated liquid B(k) to the treated liquid A(m) after the ultrafiltration process, rather than to solution X(m) that has not been subjected to the ultrafiltration process.

Therefore, the method for recovering an alkali metal salt according to this embodiment preferably includes the following step 4.
Step 4: a step of adding a remainder of the kth liquid to be treated B(k) having been mixed with the kth concentrated liquid D(k) to the mth solution X(m) (m: an integer of 2 or more) or the mth liquid to be treated A(m) after completion of step 2 for the kth solution X(k) (k: an integer of 1 or more and (N-1) or less) among the N solutions X, to the mth solution X(m) (m: an integer of (k+1) or more and N or less) or the mth liquid to be treated A(m).

FIG. 10 shows an example of carrying out step 4, which includes a step of adding the remainder of the kth treated liquid B(k) mixed with the kth concentrated liquid D(k) to the solution X(k) (m: an integer between (k+1) and N) after step 2 of the solution X(k) is completed.

Furthermore, by combining the semi-batch treatment step with the step 4 described above as the step 5 described below, the alkali metal salt can be recovered more efficiently.
Step 5: After completion of step 2 for the solution X(k), the remainder of the kth treated liquid B(k) having been mixed with the kth concentrated liquid D(k) is added to the solution X (p: an integer equal to or greater than (k+2) and equal to or less than N) or the pth treated liquid A(p).

FIG. 8 shows an example of carrying out step 5, which includes a step of adding the remainder of the kth treated liquid B(k) mixed with the kth concentrated liquid D(k) to the solution X (p: an integer equal to or greater than (k+2) and equal to or less than N) after step 2 of the solution X(k) is completed.

In Figures 1 to 3, 8 and 9, first, solution X (1) is sent to the ultrafiltration membrane unit 1 described below to obtain the treated liquid A (1). When treatment in the nanofiltration membrane unit A (2a) is completed and all of the permeated liquid A (1) has been sent to the second tank 5b, the solution in the first tank 5a is recovered in an arbitrary tank. Thereafter, solution X (2) is sent to the ultrafiltration membrane unit 1 described below to obtain the treated liquid A (2), which is stored in the first tank 5a, and the process proceeds sequentially. The process proceeds in the same manner for solutions X (3) to X (N).

In the method for recovering alkali metal salts according to this embodiment, it is preferable to carry out steps 1 and 2 on all N solutions containing alkali metal ions, i.e., the first solution X(1) to the Nth solution X(N). In addition, steps other than steps 1 and 2 may be carried out on the N solutions, for example, a reverse osmosis filtration step (step 3) or an ultrafiltration step, which will be described later.

(4) Reverse Osmosis Filtration Step The method for recovering an alkali metal salt according to this embodiment preferably includes a reverse osmosis filtration step for concentrating at least one of the permeate A obtained in the first nanofiltration step and the permeate C obtained in the second nanofiltration step.
In particular, when N solutions X are present, the method for recovering an alkali metal salt according to this embodiment preferably further includes the following step 3.
Step 3: A reverse osmosis filtration step for concentrating at least one of the kth permeate A(k) and the kth permeate C(k) in at least one solution X(k).

In the reverse osmosis filtration process, at least one of permeate A and permeate C is sent to a reverse osmosis membrane unit, and a concentrated liquid with a higher alkali metal ion concentration than permeate A or permeate C sent thereto, and a permeate with a lower alkali metal ion concentration than permeate A and permeate C are obtained.

Examples of operation control methods in the reverse osmosis filtration step include constant flow rate filtration and low pressure filtration.
In the case of constant flow rate filtration, from the viewpoint of Li ⁺ recovery efficiency, it is preferable that the permeate flow rate is 1% or more of the permeate A obtained in the first nanofiltration step or the permeate C obtained in the second nanofiltration step per minute, and from the viewpoint of ease of control, it is preferable that the permeate flow rate is 50% or less of the permeate A obtained in the first nanofiltration step or the permeate C obtained in the second nanofiltration step per minute.

(4-1) Reverse Osmosis Membrane In the reverse osmosis filtration process, any reverse osmosis membrane that does not allow alkali metal ions to pass through may be used. By using such a reverse osmosis membrane, the loss of lithium ions during the process of concentrating alkali metal ions, particularly lithium ions, is extremely small, and highly efficient recovery of lithium ions is stably achieved. The higher the ion removal rate of the reverse osmosis membrane, the better the efficiency of the process. However, since membranes with high removal rates generally have poor water permeability, it is preferable to select a membrane that takes this balance into consideration.

In particular, when solution X contains neutral molecules that are uncharged under conditions of pH 3 or less, such as boron compounds typified by boric acid, the neutral molecules are not removed in the nanofiltration process and are also contained in the permeate A and permeate C obtained in the nanofiltration process. Therefore, it is preferable to remove neutral molecules while concentrating alkali metal ions by the reverse osmosis filtration process. In other words, it is preferable that the reverse osmosis membrane does not allow alkali metal ions to pass through but allows neutral molecules to pass through. In particular, a low-removal reverse osmosis membrane that has an isopropyl alcohol removal rate of 70% or more but less than 85% when an aqueous isopropyl alcohol solution at 25°C and pH 6.5 is passed through it at an operating pressure of 0.5 MPa is preferable in that it does not allow alkali metal ions to pass through but allows neutral molecules to pass through. Here, the low-rejection reverse osmosis membrane concentrates at least one of the permeated liquid A(k) and the permeated liquid C(k), and at least one of the permeated liquid A(k) and the permeated liquid C(k) contains neutral molecules that are not charged under conditions of pH 3 or less, and the low-rejection reverse osmosis membrane is a reverse osmosis filtration membrane used in the reverse osmosis filtration process.

The reverse osmosis membrane may be made of a polymer such as cellulose acetate polymer, polyamide, sulfonated polysulfone, polyacrylonitrile, polyester, polyimide, or vinyl polymer. The reverse osmosis membrane may be made of only one material, or may be made of multiple materials. The membrane structure may be an asymmetric membrane with a dense layer on at least one side of the membrane, with gradually increasing pore sizes from the dense layer toward the inside of the membrane or toward the other side, or a composite semipermeable membrane with a very thin separation functional layer made of a different material on top of the dense layer of the asymmetric membrane.

Specific examples of composite semipermeable membranes used as reverse osmosis membranes include composite semipermeable membranes that include a substrate, a porous support membrane, and a separation functional layer. Among these, composite semipermeable membranes that include a polyamide in the separation functional layer are preferred. The separation functional layer that includes a polyamide is obtained by polycondensation of a polyfunctional amine and a polyfunctional acid halide on a porous support membrane.

(4-2) Concentration by reverse osmosis membrane In the method for recovering an alkali metal salt according to this embodiment, the reverse osmosis filtration step is preferably carried out at least once for at least one of the permeated liquid A and the permeated liquid C of the nanofiltration step. That is, it is preferably carried out at least once for at least one of the kth permeated liquid A(k) obtained in the first nanofiltration step (step 1) and the kth permeated liquid C(k) obtained in the second nanofiltration step (step 2) in the solution X(k).

It is more preferable to perform the reverse osmosis filtration step only once for the permeate C(k) obtained in the second nanofiltration step for solution X(k), in order to shorten the overall process time. Note that when the second nanofiltration step is performed multiple times, it is preferable to perform the reverse osmosis filtration step only once for the permeate C(k) obtained in the final second nanofiltration step.

As described above, when solution X contains neutral molecules that are uncharged under conditions of pH 3 or less, such as boron compounds typified by boric acid, it is preferable to concentrate the alkali metal ions while removing the neutral molecules using a low-rejection reverse osmosis membrane. In this case, it is preferable that the reverse osmosis filtration process includes a circulation process in which the concentrated liquid obtained in the reverse osmosis filtration process is mixed with the solution to be supplied to the reverse osmosis filtration process, in that the alkali metal ions can be concentrated while efficiently removing the neutral molecules. When the circulation process is included, it is preferable that the period for which the circulation process is performed is within a range until the pressure resistance value of the reverse osmosis membrane unit reaches 90% from the viewpoint of the operating pressure.

In the above case, the permeate of the low-removal reverse osmosis membrane contains neutral molecules, and therefore is not suitable for use as dilution water for the treated liquid A or the treated liquid B in the nanofiltration process. The neutral molecule concentration in the dilution water is preferably 1% or less, more preferably 0.1% or less, and even more preferably 0.01% or less, relative to the neutral molecule concentration (mg/L) in the treated liquid. By being 1% or less relative to the neutral molecule concentration (mg/L) in the treated liquid, accumulation of neutral molecules in the system can be significantly prevented when the permeate of the low-removal reverse osmosis membrane is used as dilution water to perform a semi-batch treatment process for the treated liquid. When the permeate of the low-removal reverse osmosis membrane is used as dilution water, it is preferable to send the permeate of the low-removal reverse osmosis membrane to a high-removal reverse osmosis membrane unit equipped with a high-removal reverse osmosis membrane defined below to remove neutral molecules. By passing through the high-removal reverse osmosis membrane, the neutral molecule concentration in the permeate of the low-removal reverse osmosis membrane can be set to a range suitable for the dilution water.

"High-removal reverse osmosis membrane" refers to a reverse osmosis membrane that has an isopropyl alcohol removal rate of 85 to 95% when an aqueous solution of isopropyl alcohol at 25°C and pH 6.5 is passed through it at an operating pressure of 0.5 MPa.

The examples shown in Figures 1, 5, 8 and 10 are flow diagrams of an alkali metal salt recovery process that includes a reverse osmosis filtration step for concentrating permeated liquid C and a dilution step in which the permeated liquid obtained in the reverse osmosis filtration step is used as dilution water for the liquid A to be treated. Specifically, the permeated liquid C that has permeated through the nanofiltration membrane unit B (2b) or nanofiltration membrane unit A (2a) and is stored in the fourth tank 5d or the second tank 5b is sent to the fifth tank 5e or the third tank 5c, and the liquid in the fifth tank 5e or the third tank 5c is sent to the first reverse osmosis membrane unit 3a to perform the reverse osmosis filtration step. The obtained permeated liquid is sent as dilution water for the liquid A to be treated in the first tank 5a, and the concentrated liquid is sent to the sixth tank 5f. The liquid in the sixth tank 5f can be collected in any tank.

The example shown in FIG. 9 is a flow diagram of an alkali metal salt recovery process that includes a reverse osmosis filtration process for concentrating permeated liquid C, and a dilution process in which the permeated liquid obtained in the reverse osmosis filtration process is treated with a high-removal reverse osmosis membrane and then used as dilution water for the liquid A to be treated. Specifically, the permeated liquid C that has permeated through the nanofiltration membrane unit B (2b) and stored in the fourth tank 5d is sent to the fifth tank 5e, and the liquid in the fifth tank 5e is sent to the first reverse osmosis membrane unit 3a to perform the reverse osmosis filtration process. The concentrated liquid obtained in the reverse osmosis filtration process is sent to the sixth tank 5f. The liquid in the sixth tank 5f can be collected in any tank. Furthermore, the permeated liquid obtained in the reverse osmosis filtration process is sent to the high-removal reverse osmosis membrane unit 4, and the concentrated liquid obtained in the high-removal reverse osmosis membrane unit 4 is discharged, and the permeated liquid is supplied to the first tank 5a, and can be used as dilution water for the liquid A to be treated.

The example shown in FIG. 3 is a flow diagram of an alkali metal salt recovery process in which the reverse osmosis filtration process for concentrating the permeated liquid C includes a process for circulating the concentrated liquid, and a dilution process in which the permeated liquid obtained in the reverse osmosis filtration process is treated with a high-removal reverse osmosis membrane and then used as dilution water for the liquid to be treated A. Specifically, the permeated liquid C that has permeated through the nanofiltration membrane unit B (2b) and stored in the fourth tank 5d is sent to the fifth tank 5e, the liquid in the fifth tank 5e is sent to the first reverse osmosis membrane unit 3a, and the concentrated liquid obtained in the first reverse osmosis membrane unit 3a is mixed with the fifth tank 5e while the reverse osmosis filtration process is performed. Furthermore, the permeated liquid obtained in the reverse osmosis filtration process is sent to the high-removal reverse osmosis membrane unit 4, and the concentrated liquid obtained in the high-removal reverse osmosis membrane unit 4 is discharged, and the permeated liquid is supplied to the first tank 5a, and can be used as dilution water for the liquid to be treated A. After the reverse osmosis filtration process is completed, the liquid in the fifth tank 5e, which is the concentrated liquid of the permeated liquid C, may be collected in any tank.

The example shown in FIG. 2 is a flow diagram of an alkali metal salt recovery process including a reverse osmosis filtration step for concentrating permeated liquid A, a reverse osmosis filtration step for concentrating permeated liquid C, and a dilution step for using the permeated liquid obtained in each reverse osmosis filtration step as dilution water for the liquid A to be treated. Specifically, first, the liquid in the second tank 5b that has permeated through the nanofiltration membrane unit A (2a) is sent to the fifth tank 5e. The liquid in the fifth tank 5e is sent to the first reverse osmosis membrane unit 3a, the resulting concentrated liquid is sent to the sixth tank 5f, and the permeated liquid is added to the liquid A to be treated as dilution water. Next, the liquid in the sixth tank 5f is sent to the third tank 5c. The liquid in the third tank 5c is sent to the nanofiltration membrane unit B (2b) as the liquid B to be treated, and the permeated liquid C stored in the fourth tank 5d is sent to the seventh tank 5g. The liquid in the seventh tank 5g is sent to the second reverse osmosis membrane unit 3b, the resulting concentrated liquid is sent to the eighth tank 5h, and the permeated liquid is added to the treated liquid A as dilution water. The liquid in the eighth tank 5h can be collected in any tank.

(5) Ultrafiltration Step The solution X(k) may be subjected to ultrafiltration before the first nanofiltration step. High molecular weight organic matter can be removed by ultrafiltration, and the removal of high molecular weight organic matter can suppress fouling of the nanofiltration membrane.
When a plurality of solutions are mixed to obtain solution X(k), each of these solutions may be subjected to ultrafiltration. The permeate of the ultrafiltration membrane unit is used as the treated liquid A(k) in the first nanofiltration step.
In FIGS. 1 to 3, the solution X(k) is sent to the ultrafiltration membrane unit 1, and the permeated liquid is sent to the first tank 5a.

(6) Recovery Step In this step, an alkali metal salt is recovered from the permeate C containing alkali metal ions obtained in the second nanofiltration step or a concentrate of the permeate C obtained through the reverse osmosis filtration step. The recovery step preferably includes concentrating the aqueous alkali metal salt solution.

The alkali metal salt can be recovered by a known method. For example, when the alkali metal salt is a potassium salt, the recovery is carried out by utilizing the temperature dependency of solubility or by adding a poor solvent such as ethanol.
Lithium salts have a lower solubility than other alkali metal salts. For example, sodium carbonate and potassium carbonate have a high solubility in water (20 g or more per 100 mL of water), but lithium carbonate dissolves only 1.33 g per 100 mL of water at 25° C. Therefore, lithium can be recovered as lithium carbonate by adding carbonate to permeate C or a concentrated solution of permeate C containing alkali metal ions. Since the solubility of lithium carbonate further decreases at high temperatures, the aqueous solution may be heated.

(7) Comparative Example FIGS. 4, 6 and 7 are schematic flow diagrams showing a process for recovering an alkali metal salt in a comparative example.
The difference between the process configuration of Figure 4 and the process configuration of Figure 1 is that the concentrated liquid B from the nanofiltration membrane unit A (2a) and the concentrated liquid D from the nanofiltration membrane unit B (2b) are discharged without being mixed with the liquids in the first tank 5a and the third tank 5c, respectively, and the rest is the same as in Figure 1. In the case of the above process, there is a problem in that the Li ⁺ recovery rate is low.

The process configuration of Fig. 6 is the same as the example shown in Fig. 1 except that there is no third tank 5c, nanofiltration membrane unit B (2b) or fourth tank 5d, i.e., there is no second nanofiltration step, and the liquid in the second tank 5b is sent to the fifth tank 5e. In the case of the above process, there is a problem in that the Li ⁺ purity of the recovered liquid is low.

The process configuration of FIG. 7 is a continuous treatment process, in which the solution X (1) is sent to the ultrafiltration membrane unit 1, and the permeate of the ultrafiltration membrane unit 1 is sent to the nanofiltration membrane unit A (2a) while being mixed with dilution water. The permeate A of the nanofiltration membrane unit A (2a) is sent to the nanofiltration membrane unit B (2b), and the concentrated solution B of the nanofiltration membrane unit A (2a) is collected in the first tank 5a. The permeate C of the nanofiltration membrane unit B (2b) is sent to the first reverse osmosis membrane unit 3a, and the concentrated solution D of the nanofiltration membrane unit B (2b) is mixed with the concentrated solution B of the nanofiltration membrane unit A (2a) and sent to the first tank 5a. The concentrated solution of the first reverse osmosis membrane unit 3a is sent to the fifth tank 5e. The permeate of the first reverse osmosis membrane unit 3a is used as part of the dilution water. After the entire amount of solution X(1) has been treated, solutions X(2) and X(3) are successively treated in the same manner. The above process has a problem in that the Li ⁺ recovery rate is low.

(8) Alkali metal salt recovery device The alkali metal salt recovery device of the present invention comprises:
a first separation means for separating a solution containing alkali metal ions as a treatment liquid A into a permeate liquid A and a concentrate liquid B by a first nanofiltration membrane unit;
a first circulation means for mixing the concentrated liquid B with the remainder of the liquid A to be treated;
a second separation means for separating the permeated liquid A or a concentrate of the permeated liquid A as a liquid to be treated B into a permeated liquid C and a concentrate D by a second nanofiltration membrane unit;
A second circulation means for mixing the concentrated liquid D with the remainder of the liquid to be treated B;
A dilution means for adding dilution water to at least one of the liquid A and the liquid B;
a flow rate control means capable of controlling the flow rates of the permeated liquid A and the concentrated liquid B in the first separation means, and the permeated liquid C and the concentrated liquid D in the second separation means;
The dilution means includes a flow rate control means for synchronizing the flow rate of dilution water added with the flow rate of the permeated liquid when the liquid to be treated to which the dilution water is added is sent to the nanofiltration membrane unit.

The alkali metal salt recovery apparatus of the present invention comprises:
The system includes a first separation device, a first circulation device, a second separation device, a second circulation device, a dilution device, a flow rate control device a, and a flow rate control device b,
The first separation equipment includes a first nanofiltration membrane unit, and in the first separation equipment, a liquid to be treated A, which is a solution containing alkali metal ions, is separated into a permeate A and a concentrated liquid B by the first nanofiltration membrane unit,
In the first circulation facility, the concentrated liquid B is mixed with the remainder of the treated liquid A,
In the second separation equipment, the permeated liquid A or a concentrate of the permeated liquid A is separated into a permeated liquid C and a concentrate D by a second nanofiltration membrane unit as a liquid to be treated B,
In the second circulation facility, the concentrated liquid D is mixed with the remainder of the treated liquid B,
In the dilution equipment, dilution water is added to at least one of the liquid to be treated A and the liquid to be treated B,
In the flow rate control equipment a, the flow rates of the permeate A and the concentrate B in the first separation device, and the flow rates of the permeate C and the concentrate D in the second separation device are controlled;
In the flow rate control equipment b, the flow rate of dilution water added in the dilution means is synchronized with the flow rate of the permeate when the treated liquid to which the dilution water is added is sent to the nanofiltration membrane unit.
It may be a recovery device.

In the first and second separation equipment, the nanofiltration membrane unit preferably has a pressure vessel filled with spiral elements of nanofiltration membrane, and is structured so that a solution containing alkali metal ions can be supplied to the vessel by a high-pressure pump. The nanofiltration membrane unit may have multiple vessels connected in parallel or series, and each vessel may be filled with multiple nanofiltration membrane elements. Nanofiltration membrane spiral elements of any diameter and length can be used. The size of the nanofiltration membrane spiral elements varies depending on the membrane area, and for the same membrane type, the larger the membrane area, the more liquid volume can be treated per unit time. The size and number of nanofiltration membrane spiral elements can be determined arbitrarily depending on the scale of the liquid A to be treated.

The flow rate control equipment a preferably has a meter (flow meter) capable of measuring the flow rate of the permeate and concentrate of the nanofiltration membrane unit in order to keep the flow rates of the permeate and concentrate of the nanofiltration membrane unit constant. For the flow rate control of the permeate, the high-pressure pump preferably has a mechanism that can receive data from the permeate flow meter at any time and control the output of the high-pressure pump so that the permeate flow rate is constant. For the flow rate control of the concentrate, it is preferable to have an electromagnetic valve near the concentrate flow meter, and the electromagnetic valve preferably has a mechanism that can receive data from the concentrate flow meter at any time and control the concentrate flow rate to be constant.
It is preferable that the first circulation equipment and the second circulation equipment have a tank (raw water tank) for filling the liquid to be treated, and have piping for circulating the concentrated liquid discharged from the nanofiltration membrane unit to the raw water tank.
The dilution equipment preferably has a tank (dilution water tank) for filling with dilution water, and a pump (dilution water supply pump) for supplying dilution water from the dilution water tank to the raw water tank.
As the flow rate control equipment b, the dilution water delivery pump preferably has a mechanism for receiving data from the permeate flow meter at all times and delivering dilution water at the same flow rate as the permeate flow rate.
The above-mentioned equipment is preferably made of materials that are resistant to the properties of the liquid to be treated and the operating pressure.
In order to achieve the recovery of alkali metal salts, the recovery apparatus of the present invention can, in addition to the above, select and arbitrarily combine pumps, piping, valves, tanks, vessels, temperature control devices, and instruments (pH meter, conductivity meter, flow meter, pressure gauge, etc.).

The present invention will be explained below with reference to examples, but the present invention is not limited to these examples. Measurements in the examples and comparative examples were carried out as follows.

<Performance of nanofiltration membranes and reverse osmosis membranes>
(Glucose removal rate and isopropyl alcohol removal rate of nanofiltration membrane)
The glucose concentration of the permeate and the feed water when a 1000 mg / L glucose aqueous solution at 25 ° C. and pH 6.5 as the feed water is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa, and the isopropyl alcohol concentration of the permeate and the feed water when a 1000 mg / L isopropyl alcohol aqueous solution at 25 ° C. and pH 6.5 is permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa. The isopropyl alcohol removal rate and the glucose removal rate were calculated using the following formula from the isopropyl alcohol concentration of the permeate and the feed water when the aqueous solution was permeated through the nanofiltration membrane at an operating pressure of 0.5 MPa.
Isopropyl alcohol removal rate (%) = 100 x (1 - (isopropyl alcohol concentration in permeate / isopropyl alcohol concentration in feed water))
Glucose removal rate (%)=100×(1−(glucose concentration in permeate/glucose concentration in feed water))
The isopropyl alcohol concentration was determined using a gas chromatograph (GC-18A manufactured by Shimadzu Corporation), and the glucose concentration was determined using a refractometer (RID-6A manufactured by Shimadzu Corporation).

(Magnesium sulfate removal rate and magnesium chloride removal rate of nanofiltration membrane)
The MgSO4 removal rate and the MgCl2 removal rate were calculated using the following formulas from the magnesium sulfate concentrations in the permeate and feed water when a 2000 mg/L aqueous magnesium sulfate (hereinafter also referred to as " _MgSO4 ") solution at 25°C and pH 6.5 was passed through a nanofiltration membrane at an operating pressure of 0.5 MPa as the feed water, and the magnesium chloride concentrations in the permeate and feed water when a 2000 mg _/ L aqueous magnesium chloride (hereinafter also referred to as _{" MgCl2} ") solution at 25°C and pH 6.5 was passed through the nanofiltration membrane at an operating pressure of 0.5 MPa.
The magnesium sulfate concentration and magnesium chloride concentration were determined by measuring the electrical conductivity of the feed water and the permeate using an electrical conductivity meter manufactured by Toa Denpa Kogyo Co., Ltd. to obtain the practical salinity, i.e., _MgSO4 concentration and _MgCl2 concentration, respectively.
_MgSO4 removal rate (%) = 100 × {1 - ( _MgSO4 concentration in permeate / _MgSO4 concentration in feed water)}
_MgCl2 removal rate (%) = 100 × {1 - ( _MgCl2 concentration in permeate / _MgCl2 concentration in feed water)}

(Positron annihilation lifetime measurement using positron beam method)
The average pore size was derived for nanofiltration membrane A and nanofiltration membrane B described below.
The positron annihilation lifetime measurement of the separation functional layer was carried out using a positron beam method as follows. The composite semipermeable membrane was freeze-dried under reduced pressure at -30°C and cut into 1.5 cm x 1.5 cm squares to prepare test samples. The separation functional layer side of the test sample was measured with a barium difluoride scintillation counter using a photomultiplier tube at a total count of 5 million at room temperature under vacuum using a thin film positron annihilation lifetime measurement device equipped with a positron beam generator (this device is described in detail in, for example, Radiation Physics and Chemistry, 58, 603, Pergamon (2000)). From the average lifetime τ of the third component obtained by analysis, the Tao-Eldrup equation was used to derive the average pore diameter R1 when the beam intensity was 0.1 keV and the average pore diameter R2 when the beam intensity was 0.5 keV, and R1/R2 was calculated. The obtained value was taken as the "pore diameter distribution."

(Solution X)
Lithium sulfate, nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in water so as to achieve the Li ⁺ , Ni2 ⁺ , Co2 ⁺ , and Mn2 ⁺ concentrations of rare metal-containing acidic aqueous solution A described in Table 1 of Patent Document 2, and the pH was adjusted to 1 using sulfuric acid. Furthermore, the aqueous solution was diluted 1.2-fold with an aqueous sulfuric acid solution of pH 1 to prepare solution Xa (number 1).
A solution was prepared in the same manner as in solution Xa (number 1) except that the Li ⁺ concentration was 1/2, to prepare solution Xa (number 2).
A solution was prepared in the same manner as in solution Xa (number 1) except that the Li ⁺ concentration was 1/4, to prepare solution Xa (number 3).
Solutions Xb (numbers 1 to 3) were prepared in the same manner as solution Xa (numbers 1 to 3), except that the pH was adjusted to 3.7.
Solutions Xc (numbers 1 to 3) were prepared in the same manner as solution Xa (numbers 1 to 3), except that boric acid was added to each of the solutions Xa to adjust the boron concentration to 50 mg/L.
The concentrations of various ions in the solution obtained above were quantified using a Hitachi P-4010 ICP (inductively coupled plasma emission spectrometry) apparatus. The results are shown in Table 1.
The liquid volumes of the numbers 1 to 3 constituting the solutions Xa, Xb and Xc were each 1000 L.

<Preparation of nanofiltration membrane and reverse osmosis membrane>
(Nanofiltration membrane A)
A 18.0 mass% dimethylformamide (DMF) solution of polysulfone was cast at room temperature (25°C) to a thickness of 180 μm onto a nonwoven fabric made of polyester fibers (air permeability 1 cc/ ^cm2 /s), and then immediately immersed in pure water and allowed to stand for 5 minutes to produce a porous support membrane (thickness 160 μm) made of fiber-reinforced polysulfone.

Next, the surface temperature of the porous support membrane was adjusted to 25°C while blowing air adjusted to 25°C to remove excess moisture. A 30°C aqueous solution containing 2.0% by mass of piperazine, 250 ppm of sodium dodecyl diphenyl ether disulfonate, and 1.0% by mass of trisodium phosphate was applied to the surface of the porous support membrane and left to stand for 15 seconds, after which nitrogen was blown from an air nozzle to remove excess aqueous solution, forming a coating layer of an amine aqueous solution on the porous support membrane. Furthermore, a 38°C n-decane solution containing 0.2% by mass of trimesic acid chloride (hereinafter referred to as "TMC") was uniformly applied to the entire surface of the porous support membrane, and then the membrane was left to stand for 1 minute at a relative humidity of 70% and a temperature of 25°C to perform interfacial polycondensation, and two fluids (pure water and air) were blown onto the membrane surface to remove the solution on the surface. The membrane was then washed with 80°C pure water to obtain nanofiltration membrane A.

(Nanofiltration membrane B)
Nanofiltration membrane B was produced in the same manner as nanofiltration membrane A, except that piperazine was changed to 2,5-dimethylpiperazine, and a 38°C n-decane solution containing 0.2 mass% TMC was uniformly applied to the entire surface of the porous support membrane, and then the membrane was left to stand at a relative humidity of 80% and 25°C for 1 minute.

(Nanofiltration membrane E)
The nanofiltration membrane E was SelRO (registered trademark) MPS-34 manufactured by KOCH.

The membrane performances of nanofiltration membrane A, nanofiltration membrane B and nanofiltration membrane E are shown in Table 2.
Nanofiltration membrane A, nanofiltration membrane B and nanofiltration membrane E were each spirally wound by any method and used as a membrane element having a diameter of 20.32 cm and a length of 102 cm (hereinafter referred to as "8 inch element").

(Reverse osmosis membrane C)
A porous support membrane was prepared in the same manner as the nanofiltration membrane A, and the membrane surface temperature of the porous support membrane was adjusted to 25 ° C. while removing excess water by blowing air adjusted to 25 ° C. After immersing for 15 seconds in an aqueous solution containing 5.0 mass% m-phenylenediamine (hereinafter, "m-PDA"), nitrogen was blown from an air nozzle to remove excess aqueous solution, and a 30 ° C. n-decane solution containing 0.18 mass% TMC was uniformly applied to the entire surface of the porous support membrane, and then the membrane was left to stand at 30 ° C. for 1 minute, and two fluids (pure water and air) were sprayed on the membrane surface to remove the solution on the surface. Then, it was washed with pure water at 80 ° C. to obtain a reverse osmosis membrane C.

(Reverse osmosis membrane D)
This was produced in the same manner as in the case of reverse osmosis membrane C, except that the m-PDA content was changed to 1.8% by mass and the TMC content was changed to 0.07% by mass.

The membrane performance of the reverse osmosis membrane C and the reverse osmosis membrane D is shown in Table 3.
Each of the reverse osmosis membranes C and D was spirally wound by any method and used as an 8-inch element.

The results in Table 3 show that reverse osmosis membrane C is a high-rejection reverse osmosis membrane, and reverse osmosis membrane D is a low-rejection reverse osmosis membrane.

<Evaluation of alkali metal salt recovery>
(alkali metal ion ratio)
The alkali metal ion ratio was calculated from the concentrations of various ions in the solution according to the following formula.
Alkali metal ion ratio = lithium ion concentration / (cobalt ion concentration + nickel ion concentration + manganese ion concentration)

(Li ⁺ Recovery Rate)
The Li ⁺ recovery rate was calculated by the following formula:
Li ⁺ recovery rate (%)={(amount of liquid finally concentrated in the reverse osmosis membrane unit (L))×(Li ⁺ concentration in the liquid finally concentrated in the reverse osmosis membrane unit (mg/L))}/{(initial amount of solution X (L))×Li ⁺ concentration in the initial solution of solution X (mg/L))}

In addition, in the case of recovering the remainder of the treated liquid B(k) in the second nanofiltration step in solution X(k), the Li ⁺ contained in the remainder of the treated liquid B(k) is recovered by adding it to the treated liquid A(m) (m: an integer of (k+1) or more and N or less) or the treated liquid A(p) (p: an integer of (k+2) or more and N or less), so it is considered to have been recovered, and the Li ⁺ recovery rate (k) in solution X(k) was calculated by the following formula. In Example 9, the following formula was used, but in the case of the number of solutions N=3, and also for k=2 and 3, the Li ⁺ contained in the remainder of the treated liquid B(k) in the second nanofiltration step was considered to be recovered.
Li ⁺ recovery rate (k) (%) = {(amount of liquid (L) finally concentrated in the reverse osmosis membrane unit in the treatment of solution X(k)) × (Li ⁺ concentration (mg / L) in the liquid finally concentrated in the reverse osmosis membrane unit in the treatment of solution X(k)) + (amount of liquid (L) of the remaining part of the treated liquid B(k) in the second nanofiltration step) × (Li ⁺ concentration (mg / L) in the remaining part of the treated liquid B(k) in the second nanofiltration step))} / {(initial amount of solution X(k)) × (Li ⁺ concentration (mg / L) in the initial liquid of solution X(k)) + (amount of liquid (L) of the remaining part of the treated liquid B(k-1:k≧2) in the second nanofiltration step added to solution X(k)) × ((Li ⁺ concentration (mg / L) in the remaining part of the treated liquid B(k-1:k≧2) in the second nanofiltration step added to solution X(k))}

(Li ⁺ Purity)
The Li ⁺ purity was determined as the alkali metal ion ratio in the liquid finally concentrated in the reverse osmosis membrane unit.

(Boron concentration ratio)
The boron concentration ratio was defined as the ratio of the boron concentration to the lithium ion concentration in the liquid finally concentrated in the reverse osmosis membrane unit.

(Total Processing Time)
The total treatment time was defined as the sum of the times required to complete treatment of the various solutions X, numbers 1 to 3.

Example 1
In the process configuration shown in FIG. 1, the nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), and the reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a. The half-batch treatment process was carried out in the order of solution Xa numbers 1 to 3, and the alkali metal salt was recovered. The pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration process was carried out at a constant flow rate of 60 L/min permeate flow rate. The alkali metal ion recovery rate of the nanofiltration process was confirmed by monitoring the operating pressure using formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration process was 93% and the alkali metal ion recovery rate in the second nanofiltration process was 96%, and the reverse osmosis filtration process was carried out at a constant flow rate of 60 L/min permeate flow rate and continued until the operating pressure was 7 MPa. In addition, four 8-inch elements were connected in series and used for each nanofiltration membrane unit and reverse osmosis membrane unit.
The results of this process are shown in Table 4.

Example 2
The alkali metal salt recovery process was carried out in the same manner as in Example 1, except that solutions Xb Nos. 1 to 3 were used instead of solution Xa.
The results of carrying out this process are shown in Table 4. When the pH was as high as 3.7, the permeability of alkali metal ions decreased and the total treatment time increased compared to Example 1 where the pH was 1.0, but lithium ions could be recovered with high purity and high recovery rate.

Example 3
In the process configuration shown in FIG. 2, the nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), and the reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a and the second reverse osmosis membrane unit 3b. The half-batch treatment process was carried out in the order of solution Xa numbers 1 to 3, and the alkali metal salt recovery process was carried out. The pressure resistance value of the second reverse osmosis membrane unit 3b was 8 MPa. The nanofiltration process was carried out at a constant flow rate of 60 L/min permeate flow rate. The alkali metal ion recovery rates of the first nanofiltration process and the second nanofiltration process were confirmed by monitoring the operating pressure using the formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration process was 93% and the alkali metal ion recovery rate in the second nanofiltration process was 96%. The reverse osmosis filtration process was carried out at a constant flow rate of 15 L/min permeate flow rate, and continued until the operating pressure reached 7 MPa. Each nanofiltration membrane unit and reverse osmosis membrane unit used four 8-inch elements connected in series.

The results of this process are shown in Table 4. When the reverse osmosis membrane unit was used for concentration after each nanofiltration step, the total processing time increased slightly compared to when RO concentration was used only once after the subsequent nanofiltration step, but lithium ions could be recovered with high purity and high recovery rate.

Example 4
The alkali metal salt recovery process was carried out in the same manner as in Example 1, except that nanofiltration membrane B was used.
The results of carrying out this process are shown in Table 4. It can be seen that when nanofiltration membrane B satisfying the prescribed performance was used, the purity and recovery rate were improved and processing could be carried out in a short time.

Example 5
The alkali metal salt recovery process was carried out in the same manner as in Example 4, except that the alkali metal ion recovery rate in the nanofiltration step was confirmed by appropriately analyzing the permeate without monitoring the operating pressure using equation (2).
The results of carrying out this process are shown in Table 4. Since appropriate analysis requires time, the processing time was increased compared to the case where the operating pressure was monitored using equation (2).

(Example 6)
The alkali metal salt recovery process was carried out in the same manner as in Example 4, except that solution Xc was used.
The results of this process are shown in Table 4.

(Example 7)
In the process configuration shown in FIG. 9, the nanofiltration membrane B was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), the reverse osmosis membrane D was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and the reverse osmosis membrane D was used as the reverse osmosis membrane of the high-removal reverse osmosis membrane unit 4. A half-batch treatment process was carried out in the order of solutions Xc1 to 3, and an alkali metal salt recovery process was carried out. The first nanofiltration step and the second nanofiltration step were carried out at a constant flow rate of 60 L/min permeate flow rate. The alkali metal ion recovery rate of the first nanofiltration step and the second nanofiltration step was confirmed by monitoring the operating pressure using formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration step was 93% and the alkali metal ion recovery rate in the second nanofiltration step was 96%. The reverse osmosis filtration step was carried out at a constant flow rate of 60 L/min permeate flow rate and continued until the operating pressure reached 7 MPa. Each nanofiltration membrane unit and reverse osmosis membrane unit used four 8-inch elements connected in series.
The results of carrying out this process are shown in Table 4. It can be seen that boron was successfully removed by concentrating using reverse osmosis membrane D, which is a low-removal membrane.

(Example 8)
In the process configuration shown in FIG. 3, the nanofiltration membrane B was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), the reverse osmosis membrane D was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and the reverse osmosis membrane D was used as the reverse osmosis membrane of the high-removal reverse osmosis membrane unit 4. A half-batch treatment process was carried out in the order of solutions Xc1 to 3, and an alkali metal salt recovery process was carried out. The first nanofiltration step and the second nanofiltration step were carried out at a constant flow rate of 60 L/min permeate flow rate. The alkali metal ion recovery rate of the first nanofiltration step and the second nanofiltration step was confirmed by monitoring the operating pressure using formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration step was 93% and the alkali metal ion recovery rate in the second nanofiltration step was 96%, and the reverse osmosis filtration step was carried out at a constant flow rate of 60 L/min permeate flow rate, and continued until the operating pressure reached 7 MPa. Each nanofiltration membrane unit and reverse osmosis membrane unit used four 8-inch elements connected in series.
The results of carrying out this process are shown in Table 4. It can be seen that by concentrating using the reverse osmosis membrane D, which is a low-removal membrane, and providing a circulation step in the reverse osmosis filtration step, more boron was removed than in Example 7.

Example 9
In the process configuration shown in FIG. 8, the nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a) and the nanofiltration membrane unit B (2b), and the reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a. The half-batch treatment process was carried out in the order of solution Xa numbers 1 to 3, and the alkali metal salt was recovered. At this time, the treated liquid B (1) after step 2 in solution Xa1 (i.e., solution X (1)) was added to the treated liquid A (3) in solution Xa3 (i.e., solution X (3)). The pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration process was carried out at a constant flow rate of filtration with a permeate flow rate of 60 L/min. The alkali metal ion recovery rate in the nanofiltration step was confirmed by monitoring the operating pressure using equation (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration step was 93% and in the second nanofiltration step was 80%, while the reverse osmosis filtration step was performed at a constant flow rate of 60 L/min for the permeate and continued until the operating pressure reached 7 MPa. Each nanofiltration membrane unit and reverse osmosis membrane unit used four 8-inch elements connected in series.
The results of carrying out this process are shown in Table 4. It can be seen that adding the remainder of the treated liquid B(k) to the treated liquid A(p) makes it possible to recover lithium ions with higher purity and higher recovery rate.

(Comparative Example 1)
In the process configuration shown in Figure 4, nanofiltration membrane A was used as the nanofiltration membrane of nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and a semi-batch treatment process was carried out in the order of numbers 1 to 3 for solution Xa, to carry out an alkali metal salt recovery process. Note that in all steps, the permeate flow rate was 60 L/min. Note that four 8-inch elements were connected in series and used for each nanofiltration membrane unit and reverse osmosis membrane unit.
The results of carrying out this process are shown in Table 5. It can be seen that when the nanofiltration step does not have a recycling step, the lithium recovery rate is low.

Example 10
In the process configuration shown in FIG. 5, the nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a), and the reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and the alkali metal salt recovery process was carried out in the order of solution Xa numbers 1 to 3. The pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration process was carried out at a constant flow rate of 60 L/min permeate flow rate. The alkali metal ion recovery rate of the nanofiltration process was confirmed by monitoring the operating pressure using formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration process was 93% and the alkali metal ion recovery rate in the second nanofiltration process was 80%, and the reverse osmosis filtration process was carried out at a constant flow rate of 60 L/min permeate flow rate and continued until the operating pressure was 7 MPa. In addition, the process was carried out so that the permeate flow rate was 60 L/min in all processes. In addition, four 8-inch elements were connected in series and used for each nanofiltration membrane unit and reverse osmosis membrane unit.
The results of this process are shown in Table 5.

(Comparative Example 2)
In the system configuration shown in Figure 6, nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a), and reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and a semi-batch treatment process was carried out in the order of numbers 1 to 3 for solution Xa, to carry out an alkali metal salt recovery process. Note that in all steps, the permeate flow rate was set to 60 L/min. Note that each nanofiltration membrane unit and reverse osmosis membrane unit used four 8-inch elements connected in series.
The results of carrying out this process are shown in Table 5. It can be seen that with only the first nanofiltration step, the lithium purity is low.

(Comparative Example 3)
In the process configuration shown in Figure 7, nanofiltration membrane A was used as the nanofiltration membrane of nanofiltration membrane unit A (2a) and nanofiltration membrane unit B (2b), and reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and continuous treatment steps were carried out in the order of numbers 1 to 3 for solution Xa, to carry out the alkali metal salt recovery process. Note that in all steps, the permeate flow rate was set to 60 L/min. Note that four 8-inch elements were connected in series and used for each nanofiltration membrane unit and reverse osmosis membrane unit.
The results of this process are shown in Table 5. It can be seen that the lithium recovery rate is low in the continuous processing step.

(Comparative Example 4)
An alkali metal salt recovery process was carried out in the same manner as in Comparative Example 1, except that nanofiltration membrane E was used.
The results of this process are shown in Table 5.

(Example 11)
The process for recovering an alkali metal salt was carried out in the same manner as in Example 1, except that nanofiltration membrane E was used.
The results of carrying out this process are shown in Table 5. It can be seen that by applying the process according to this embodiment, the lithium recovery rate is improved compared to Comparative Example 4.

Example 12
The alkali metal salt recovery process was carried out in the same manner as in Example 9, except that nanofiltration membrane E was used.
The results of carrying out this process are shown in Table 5. It can be seen that even when the nanofiltration membrane E was used, lithium ions can be recovered with higher purity and higher recovery rate by adding the remainder of the treated liquid B(k) to the treated liquid A(p).

(Example 13)
In the process configuration shown in FIG. 10, the nanofiltration membrane A was used as the nanofiltration membrane of the nanofiltration membrane unit A (2a), and the reverse osmosis membrane C was used as the reverse osmosis membrane of the first reverse osmosis membrane unit 3a, and the alkali metal salt recovery process was carried out in the order of solution Xa numbers 1 to 3. The pressure resistance value of the first reverse osmosis membrane unit 3a was 8 MPa. The nanofiltration process was carried out at a constant flow rate of 60 L/min permeate flow rate. The alkali metal ion recovery rate of the nanofiltration process was confirmed by monitoring the operating pressure using formula (2), and filtration was continued until the alkali metal ion recovery rate in the first nanofiltration process was 93% and the alkali metal ion recovery rate in the second nanofiltration process was 80%, and the reverse osmosis filtration process was carried out at a constant flow rate of 60 L/min permeate flow rate and continued until the operating pressure was 7 MPa. In addition, the permeate flow rate was 60 L/min in all processes. Each nanofiltration membrane unit and reverse osmosis membrane unit used four 8-inch elements connected in series.
The results of carrying out this process are shown in Table 5. It can be seen that by adding the remainder of the treated liquid B(k) to the treated liquid A(m), lithium ions can be recovered with higher purity and higher recovery rate.

 

 

In the above examples and comparative examples, the higher the lithium recovery rate and lithium purity, the more advantageous it is, while the lower the boron concentration ratio and total treatment time, the more advantageous it is.
From the above results, it was found that Examples 1 to 8, which are the alkali metal salt recovery methods of the present invention, are capable of recovering alkali metal salts with high purity, at a high recovery rate, and in a short time, as compared with Comparative Examples 1 to 4. Furthermore, from the results of Examples 6 to 8, it was found that even when neutral molecules such as boron are contained, these can be removed with high efficiency and the total treatment time can be shortened.

Although the present invention has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2022-192379) filed on November 30, 2022, and is incorporated by reference in its entirety. In addition, all references cited herein are incorporated in their entirety.

The present invention can be suitably used as a method for efficiently separating and recovering alkali metals such as lithium from lithium-ion batteries and waste materials, waste liquids, ores, slag, etc. generated during the manufacturing process of the batteries.

1 Ultrafiltration membrane unit 2a Nanofiltration membrane unit A
2b Nanofiltration membrane unit B
3a First reverse osmosis membrane unit 3b Second reverse osmosis membrane unit 4 High-removal reverse osmosis membrane unit 5a First tank 5b Second tank 5c Third tank 5d Fourth tank 5e Fifth tank 5f Sixth tank 5g Seventh tank 5h Eighth tank

Claims (15)

A method for recovering an alkali metal salt, comprising the following steps 1 and 2:
Step 1: A first nanofiltration step in which a solution X containing alkali metal ions is sent to a nanofiltration membrane unit A as a treated liquid A, and separated into a permeated liquid A and a concentrated liquid B, and further, the concentrated liquid B is mixed with the remainder of the treated liquid A and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid A.
Step 2: The permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to the nanofiltration membrane unit A as the liquid to be treated B, and separated into the permeated liquid C and the concentrate D. The concentrate D is further mixed with the remaining part of the liquid to be treated B and sent again to the nanofiltration membrane unit A to further obtain the permeated liquid C. Or,
The permeated liquid A or a concentrate of the permeated liquid A obtained in the step 1 is sent to a nanofiltration membrane unit B as a liquid to be treated B, and separated into a permeated liquid C and a concentrate D. The concentrate D is further mixed with the remainder of the liquid to be treated B and sent again to the nanofiltration membrane unit B to further obtain the permeated liquid C. A second nanofiltration step. A method for recovering an alkali metal salt, comprising the steps of: obtaining the permeate C from the solution X through the steps 1 and 2, and sequentially carrying out the steps on N solutions X (N: an integer of 2 or more),
The step 2 uses the nanofiltration membrane unit B,
2. The method for recovering an alkali metal salt according to claim 1, wherein, while carrying out step 1 and then step 2 on a k-th solution X(k) (k: an integer of 1 or more and (N-1) or less) among the N solutions X, step 1 is carried out in parallel on a (k+1)-th solution X(k+1). The method for recovering alkali metal salts according to claim 1 or 2, further comprising a step of diluting at least one of the treated liquid A and the treated liquid B. The method for recovering an alkali metal salt according to claim 2, further comprising the following step 3:
Step 3: A reverse osmosis filtration step for concentrating at least one of the kth permeate A(k) and the kth permeate C(k) in at least one of said solutions X(k). The method for recovering an alkali metal salt according to claim 4, wherein step 3 is carried out only once on the permeate C(k). The method for recovering an alkali metal salt according to claim 1 or 2, wherein the solution X has a pH of 4 or less. The method for recovering an alkali metal salt according to claim 1 or 2, wherein the alkali metal ions include lithium ions. The method for recovering an alkali metal salt according to claim 1 or 2, comprising the following step 4:
Step 4: a step of adding a remainder of the kth liquid to be treated B(k) having been mixed with the kth concentrated liquid D(k) to the mth solution X(m) (m: an integer of 2 or more) or the mth liquid to be treated A(m) after completion of step 2 for the kth solution X(k) (k: an integer of 1 or more and (N-1) or less) among the N solutions X, to the mth solution X(m) (m: an integer of (k+1) or more and N or less) or the mth liquid to be treated A(m). The nanofiltration membrane of at least one of the nanofiltration membrane unit A and the nanofiltration membrane unit B has a porous support membrane and a separation functional layer,
A positron beam is irradiated from the surface of the nanofiltration membrane on the side of the separation functional layer, and the average pore size R1 and the average pore size R2 of the separation functional layer derived by a positron annihilation lifetime measurement method satisfy 0.90≦R1/R2≦1.10.
3. The method for recovering an alkali metal salt according to claim 1 or 2.
R1: Average pore diameter when the positron beam intensity is 0.1 keV R2: Average pore diameter when the positron beam intensity is 0.5 keV 3. The method for recovering an alkali metal salt according to claim 1 or 2, wherein at least one of the steps 1 and 2 is carried out at a constant permeation flow rate, and at least one of the steps 1 and 2 is terminated when a recovery rate A (%) of the alkali metal ion reaches a target value based on the following formula (2) while monitoring a change over time of the operation pressure:

[In formula (2), A is the alkali metal ion recovery rate (%), P is the operating pressure (Pa), P _is the initial operating pressure (Pa), V _is the initial liquid volume to be treated (m ³ ), R is the alkali metal ion removal rate (%) of the nanofiltration membrane, S is the liquid recovery rate (%) of the nanofiltration process, Q _F is the supply flow rate (m ³ /s), Q _c is the concentrated liquid flow rate (m ³ /s), and t is the end time of filtration (t=tb)] The method for recovering an alkali metal salt according to claim 4 or 5, wherein at least one of the permeated liquid A(k) and the permeated liquid C(k) contains neutral molecules that are not charged under conditions of pH 3 or less, and the reverse osmosis filtration membrane used in the reverse osmosis filtration step is a low-removal reverse osmosis membrane that has an isopropyl alcohol removal rate of 70% or more and less than 85% when an aqueous isopropyl alcohol solution at pH 6.5 at 25°C is passed through the membrane at an operating pressure of 0.5 MPa. The method for recovering an alkali metal salt according to claim 11, wherein the neutral molecule is a boron compound. The method for recovering alkali metal salts according to claim 12, further comprising a circulation step in which the concentrated solution obtained in the reverse osmosis filtration step is mixed with the solution to be supplied to the reverse osmosis filtration step in the step 3. The method for recovering an alkali metal salt according to claim 11, further comprising a step of feeding the permeate obtained in the reverse osmosis filtration step to a high-removal reverse osmosis membrane unit equipped with a high-removal reverse osmosis membrane having an isopropyl alcohol removal rate of 85% to 95% when an aqueous isopropyl alcohol solution having a pH of 6.5 at 25°C is passed through the membrane at an operating pressure of 0.5 MPa, and adding the permeate obtained as dilution water to at least one of the treated liquid A and the treated liquid B. a first separation means for separating a solution containing alkali metal ions as a treatment liquid A into a permeate liquid A and a concentrate liquid B by a first nanofiltration membrane unit;
a first circulation means for mixing the concentrated liquid B with the remainder of the liquid A to be treated;
a second separation means for separating the permeated liquid A or a concentrate of the permeated liquid A as a liquid to be treated B into a permeated liquid C and a concentrate D by a second nanofiltration membrane unit;
A second circulation means for mixing the concentrated liquid D with the remainder of the liquid to be treated B;
A dilution means for adding dilution water to at least one of the liquid A and the liquid B;
a flow rate control means capable of controlling the flow rates of the permeated liquid A and the concentrated liquid B in the first separation means, and the permeated liquid C and the concentrated liquid D in the second separation means;
An alkali metal salt recovery apparatus comprising: a flow control means for synchronizing the flow rate of dilution water added in the dilution means with the flow rate of the permeated liquid when the treated liquid to which the dilution water is added is sent to a nanofiltration membrane unit.

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